The present invention relates to the general field of mixers for mixing concentric gas streams from a bypass turbine engine. The invention relates more particularly to a daisy-type mixer for a confluent-flow nozzle.
Sound pollution has nowadays become one of the main concerns of engine manufacturers, who are being confronted more and more with the sound nuisance of their turbine engines. Sources of noise in a turbine engine are numerous, but it has been found that the jet noise at the outlet from the nozzle is the noise that predominates during the stage in which an airplane is taking off. Certification authorities are becoming more and more demanding in terms of sound emissions from turbine engines, and engine manufacturers have been requested to make effort to reduce the noise from their turbine engines, and in particular the jet noise at the outlets from a nozzle.
In known manner, a confluent-flow nozzle of a turbine engine comprises a main cap, a secondary cap, and a central body, all of which are centered on the longitudinal axis of the turbine engine. The secondary cap is arranged coaxially around the primary cap so as to define an annular channel between these caps for the flow of a cold stream coming from the turbine engine. Likewise, the central body is arranged coaxially inside the primary cap so as to define an annular channel between these elements for the flow of a hot stream coming from the turbine engine.
Usually, a confluent-flow nozzle also has a mixer that is mounted at the downstream end of the primary cap so as to extend it. Such a mixer is intended to reduce jet noise at the outlet from the nozzle by forcing mixing between the cold stream and the hot stream prior to their ejection. It is well known that noise reduction can be obtained by encouraging such mixing between the two streams from the turbine engine.
Mixers for confluent-flow nozzles include in particular a so-called “daisy” mixer in which the downstream end of the primary cap of the nozzle is terminated by a substantially sinusoidal portion with inner lobes and outer lobes arranged all around the circumference of the cap.
With this type of mixer, the inner lobes form troughs that guide the cold stream radially towards the channel in which the hot stream is flowing, and the outer lobes form other troughs that guide the hot stream radially towards the channel in which the cold stream is flowing. Thus, at the outlet from the mixer, the streams coming from the turbine engine are mixed together in shear in a direction that is essentially radial. This mixing makes it possible to generate turbulence about an axis of rotation that is generally axial and of intensity that depends mainly on the ejection conditions of the streams (bypass ratio of the turbine engine, shear between the hot and cold streams) and on the conditions with which the bottoms of the lobes of the mixer are fed.
Unfortunately, the intensity of the turbulence created by a mixer of this type is not always sufficient for obtaining genuinely effective mixing between the cold and hot streams, thereby putting a limit on the reduction of the noise levels of the resulting jet, in particular the jet that is obtained during airplane takeoff stages.